Conventionally, some disk storage devices using a head for read and write operations on a disk or the like perform virtual circular control so as to control the position of the head on a circular orbit (virtual circle) that is independent of the eccentricity of the disk. In such virtual circular control, a position orbit (virtual circular orbit) is provided so as to ignore the eccentricity of the disk, and the control of the head is performed based on a value obtained by subtracting a positional fluctuation caused by the eccentricity of the disk from the position (decoded position) of the head demodulated from servo patterns for detecting the offset position with respect to the center of the disk.
In such virtual circular control, because the control is performed such that the eccentricity of the disk is disregarded, a relative velocity occurs between the head and the disk. Therefore, the head traverses the servo patterns diagonally, whereby an error occurs in the decoded position. Accordingly, in such virtual circular control, it is desired to reduce the error in the decoded position.